1. Field of Invention
This invention relates to elastomeric bearings and more particularly to elastomeric bearings which are used to support a helicopter blade from a helicopter rotor hub for rotation therewith about an axis of rotation and so that centrifugal loads generated by the blade are passed through the elastomeric bearing to the hub and so that the elastomeric bearing permits the blade to move with respect to the hub in such motions as pitch change, flapping, and possibly lead-lag. The elastomeric bearing may be of the spherical, cylindrical or other types, or combinations thereof.
When elastomeric bearings were first used in helicopter rotors, they were subjected to loads and motions beyond the experience of the art and it was found that elastomeric bearings were not proving to be as durable as had been hoped in the helicopter rotor environment. Our analysis of the problem revealed that the early elastomeric bearings used in rotor heads were failing due to bending and hoop stresses generated in the metallic shims, which caused the shims to rupture and the bearing to fail. Further investigation revealed that, quite surprisingly, these excessive shim stresses were caused by eccentric moments imposed on the shim. Still further analysis revealed that these shim moments were caused by the fact that the load vectors on all shims were not in alignment since the applied load vectors and the load reaction vectors were not in alignment in passing through the elastomeric bearing. This problem and its consequences had not been recognized in the elastomeric bearing industry until our analysis revealed it and solutions to the problem are accordingly not found in the prior art.
2. Description of the Prior Art
In the elastomeric bearing art, the problems of sizing and shaping the bearings so that they can withstand the loads and motions imparted thereto have been addressed, as have the problems of preventing lateral displacement or bulging of the shim stack, however, none of the prior art recognized the problems of excessive shim bending and hoop stresses due to the creation of moments in the shim by failing to keep the applied load vectors and the load reaction vectors, and hence the individual shim load vectors, in alignment in imparting blade centrifugal loads to the hub through the elastomeric bearing.
Hinks U.S. Pat. No. 3,080,065 shows several elastomeric or laminated bearing constructions and suggests constructions which would be laterally stable but does not address the problem of the elimination of laminate moments of concern herein and, in fact, if loads were applied to the Hinks elastomeric bearing construction of FIG. 9 as shown by reference numerals 93 and 94 therein, the laminate moments of concern herein would indeed be established.
Irwin U.S. Pat. No. 3,504,902 addresses itself to achieving lateral or stack stability in an elastomeric bearing by passing pins 23 through the laminates. This Irwin patent does not address itself to the elimination of laminate moments. It should be noted that the stress concentrations which would be created by manufacturing holes in the elastomeric bearing laminates to permit the passing of pins 23 therethrough would be considered structurally undesirable in the helicopter rotor environment.
Wildhaber U.S. Pat. No. 3,752,766 discusses shear stresses in elastomeric bearing laminates but he is merely describing the operation of the conventional elastomeric bearing in which each of the laminates experiences a certain amount of torsional shear motion due to an externally applied rotation, while supporting the total load being passed through the bearing. Wildhaber suggests that in the design of an elastomeric bearing the shear stresses in the elastomeric laminates must not be excessive but does not address himself to the eccentric moments which cause metallic laminate bending and stresses, and therefore does not address himself to the elimination of these moments.
Hinks U.S. Pat. No. 2,900,182 is also primarily directed to columnar or stack stability in an elastomeric bearing and while he shows a spherical bearing in FIG. 8, and cylindrical bearings in FIGS. 4 and 7, he does not address himself to the elimination of laminate moments which will create hoop and bending stresses and hence reduce bearing life. It should be noted that in the Hinks FIG. 4 construction, a centering pin is used to effect lateral or stack stability and some such provision would also be needed in the FIG. 7 construction if that stack were built high enough.
Hinks U.S. Pat. No. 3,228,673 is directed to the problem or providing lateral stability in the laminate column in an elastomeric bearing but does not address itself to the elimination of moments in the laminates and the life reducing hoop and bending stresses which they form therein.
Peterson U.S. Pat. No. 3,292,711 addresses itself to an elastomeric bearing constructed so as to produce column stability. The Peterson patent does not concern itself with the creation of moments in the elastomeric bearing laminates and testing therof demonstrated that endurance reducing laminate eccentric moments were present. It is important to note that column stability is important in an elastomeric bearing and that the elastomeric bearing constructions taught herein achieve column stability without introducing these undesirable eccentric moments in the elastomeric bearing laminates.